Much like knitting in the front and back of a stitch, purling in the front and back, or pfb, is a way to increase stitches easily.
Purling in the front and back is not quite as intuitive as knitting in the front and back, though the technique is basically the same.
Begin by purling the stitch through the front loop as normal, leaving the stitch on the left hand needle. To purl through the back loop, loop the right hand needle around so you're working through the back loop from right to left.
This feels pretty awkward the first couple of times you do it, but once you get the hang of it you'll be increasing on the purl side in no time.